1. Field of the Invention
This invention is directed to a process for the regeneration of the devitalized catalyst/absorber after extended exposure to pollutants in the combustion gases of engines.
2. Related Information
Turbine power plants are becoming the standard for generating electricity because they are so efficient compared to any other form of power manufacture. Turbine power plants that burn methane to produce power for residents and manufacturing facilities in cities also produce carbon monoxide and nitrogen oxide as pollutants. It is highly desirable to reduce or eliminate these pollutants so that the air is not contaminated as a result of power production.
Over the years, as the technology improved, the level of permitted pollution has been decreased. Initially, the permitted level of pollution by power plants for nitrogen oxides (NOx), which includes nitric oxide (NO) and nitrogen dioxide (NO.sub.2), was less than 100 parts-per-million (ppm) and the level of carbon monoxide (CO) was less than 100 ppm. Later, the requirements were made more stringent and it was necessary to reduce the NOx to less than 25 ppm and the CO today is still permitted at any amount less than 100 ppm. Using current technology, the output levels of NOx can be reduced to the range of 5 to 9 ppm plus slippage resulting from the selective catalytic reduction (SCR) technology described below.
The only prior technology which is currently available to obtain the 5 to 9 ppm NOx levels is called selective catalytic reduction in which ammonia is mixed with the flue gas and then passed over a catalyst which selectively combines the nitrogen oxides and ammonia to eliminate a major portion of the NOx. One problem with the selective catalytic reduction is that, as a practical matter, it is only capable of reducing the Nox to the range of 5 to 9 ppm. Another problem, referred to as slippage, is that the ammonia injected into the system to react with the NOx slips past the catalyst without conversion and is ejected from the system in its native form, which is hazardous to the environment in its own right.
There have been other technologies for reduction of pollution which have been advanced, such as overwatering in the combustor, and these also have the potential to reduce the NOx pollution, but none of them reduce the NOx to levels much less than 5 to 9 ppm.
In commonly assigned U.S. Pat. No. 5,650,127, which is incorporated herein in its entirety, we described a pollution reduction process and apparatus in which the pollutants from a turbine gas stream including No and CO in the gas stream are first oxidized to corresponding NO.sub.2 and CO.sub.2, and then the NO.sub.2 is absorbed on an absorption bed.
In commonly assigned U.S. Pat. No. 5,451,558, which is incorporated herein in its entirety, a catalyst/absorber is described and consists of a support with an alumina washcoat disposed thereover, a platinum catalyst disposed on the washcoat, and with an alkali or alkaline earth carbonate or bicarbonate coating thereon, the carbonate coating being lithium, sodium, potassium or calcium carbonate. This application also discloses a process for treating exhaust streams in which the stream is contacted with the catalyst/absorber which oxidizes the nitrogen oxides to nitrogen dioxide; oxidizes the carbon monoxides to carbon dioxide; and oxidizes the sulfur dioxide (SO.sub.2) to sulfur trioxide (SO.sub.3). This oxidation occurs at temperatures in the range of 150.degree. to about 750.degree. F., and more preferably in the range of 175.degree. to 400.degree. F., and most preferably in the range of 200.degree. to 365.degree. F. The space velocity (GHSV) of the exhaust gas may be in the range of 5,000 to 50,000 hr.sup.-1. The same catalyst/absorber has a second function of absorbing the oxidized pollutants at the same temperatures so that the resultant exhaust gas stream is substantially free of harmful pollutants.
When the catalyst/absorber ceases to be effective, and specifically, when the level of pollutants emanating from the apparatus after contact with the catalyst/absorber increases beyond an acceptable level, the absorber can be replaced, and the used absorber should then be recharged to an effective status again. One method of regenerating the catalyst is to remove the spent (saturated or partially saturated) carbonate from the catalyst/absorber and replace the spent carbonate with fresh unreacted carbonate, for example, dissolving the absorber, generally potassium carbonate or sodium carbonate, from the absorber/catalyst to remove the absorber from the catalyst, and then replacing the absorber on the catalyst with fresh absorber. The nitrates and nitrites are then separated from the unreacted carbonate in the dissolved absorber so the unreacted carbonate can be reused. However this process would most likely require removal of the catalyst/absorber from the exhaust system and create large quantities of liquid waste streams to dispose of.
U.S. Pat. No. 5,599,758 discloses hydrogen and/or carbon monoxide in an inert carrier gas passed over a devitalized catalyst/carbonate or bicarbonate absorber which has absorbed or adsorbed nitrates and nitrites from engine exhaust is to restored to the carbonate form and regenerated for reuse.